The present invention relates to processes for consolidation and densification of multiple-phase composite materials, including fibrous monolith composites.
The process of fabricating high strength materials from powders such as ceramic and metal powders generally involves preparing xe2x80x9cgreenxe2x80x9d materials that include the powder and a thermoplastic binder of variable composition. As part of the fabrication process, the binder typically is removed from the material in a binder burnout step and the powder consolidated and densified in order to obtain a final structure having the desired properties, including strength and hardness. Methods of consolidation and densification include sintering processes such as, uniaxial hot pressing, hot isostatic pressing, overpressure sintering and atmospheric (pressureless) sintering.
Sintering processes, are critical in the fabrication of materials from ceramic and metal powders. Equipment used in pressure sintering processes including hot isostatic pressing (HIP) and uniaxial hot pressing must be designed to accommodate the high temperatures and high pressures associated with these sintering methods. Purchase, operation and maintenance costs for the HIP and uniaxial hot press equipment may be high as a result of the need to incorporate vessels capable of withstanding high pressures or hydraulic controlled rams into their respective designs. There are also additional costs in addressing safety requirements and designs for the safe and reliable operation of high pressure equipment. Additionally, the capacity of HIP and uniaxial hot press equipment is limited by these requirements. Thus, production volume capabilities are reduced, which further increases production costs. Furthermore, pressing is generally limited and cannot be used effectively with three-dimensional structures having more complex geometries.
Pressureless sintering furnaces generally are less expensive to purchase, operate and maintain as compared to equipment for pressure sintering. They also provide larger production volume capabilities and lower overall production costs. However, an important disadvantage associated with pressureless sintering is the potential inability to achieve effective sintering of a material in the absence of pressure.
Fibrous monoliths (FMs) are a unique class of structural ceramics. FMs are monolithic ceramics that are manufactured by powder processing techniques using inexpensive raw materials. Methods of preparing FM filaments are known. U.S. Pat. No. 5,645,781 describes methods of preparing FM composites by extrusion of filaments having controlled texture. As a result of the combination of relatively low costs of manufacture and benefits of enhanced materials performance, FMs have been used in a wider range of applications than heretofore typical for ceramics. Fibrous monoliths typically have been formed to various fibrous textures. For example, FM filaments have been woven into thin, planar structures. Alternatively, the filaments have been formed into three-dimensional structures having complex geometries.
Generally, the macroarchitecture of an FM composite includes a plurality of filaments each including a primary phase in the form of elongated polycrystalline cells surrounded by at least a thin secondary phase in the form of a cell boundary. The material selected for the cell phase differs from the material selected for the cell boundary phase in type and/or composition. Thus, the various materials comprising a FM composite each have different material properties.
This xe2x80x9cmulti-phasexe2x80x9d nature of FM composites, along with the possibility that the composites are formed into complex structures, can increase the difficulties encountered when attempting to sinter such composites. Significantly, when two or more materials are used and are to be maintained essentially separate from each other in a composite component, the ability to effectively sinter the FM composite component can be severely limited or even prevented. Because the material properties of the two phases differ, the range of physical and chemical conditions that lead to effective sintering of the composite can be restricted. The difficulty in identifying an effective sintering regime increases further as additional materials are included in the composite. Moreover, the potential for unfavorable interactions between materials that can limit sinterability increases as additional materials are added to the composite.
There remains a need for more efficient, cost-effective sintering processes that can be utilized during fabrication of fibrous monolith composite structures, particularly those having complex geometries.
The present invention overcomes the problems encountered in conventional methods by providing efficient, cost-effective processes for consolidation and densification of composites formed of more than one composition. More specifically, the present invention provides methods of pressureless sintering that are effective for sintering fibrous monolith composite structures, including those having complex geometries. Pressureless sintering of FM composites provides for the consolidation and densification of two- and three-dimensional components in less time and at a lower cost as compared to other sintering processes. Additionally, FM composites with geometries too complicated to be processed by uniaxial hot press techniques can be sintered in accordance with the method of the present invention.
The present invention relates to methods of consolidating and densifying ceramic composite components by pressureless sintering. Components that can be consolidated and densified in accordance with the invention include those formed of composites that have two or more materials present in essentially separate phases. Such composites include fibrous monolith (FM) composites, which are made up of a plurality of filaments having a core phase that is surrounded by a shell phase.
In a pressureless sintering process, composites are heated to high temperatures without high pressure in a large volume, high temperature furnace. In comparison to various pressure sintering processes, pressureless sintering significantly lowers the overall production cost of FM composites, in part due to lower equipment purchase, operation and maintenance costs. Pressureless sintering also provides large production volume capabilities, so that mass production of FM components is possible. The processes of the present invention thus provide increased effectiveness and efficiencies in the overall fabrication of FM composite components.
As used herein. xe2x80x9cfibrous monolithic compositexe2x80x9d and xe2x80x9cfibrous monolithxe2x80x9d are intended to mean a ceramic composite material that includes a plurality of monolithic fibers, or filaments, each having at least a cell phase surrounded by a boundary phase but may include more than one core and/or shell phase. Fibrous monoliths exhibit the characteristic of non-brittle fracture, such that they provide for non-catastrophic failure.
As used herein, xe2x80x9ccell phasexe2x80x9d is intended to mean a centrally located primary material of the monolithic fiber that is dense, relatively hard and/or strong. The cell phase extends axially through the length of the fiber, and, when the fiber is viewed in cross-section, the cell phase forms the core of the fiber. The xe2x80x9ccell phasexe2x80x9d also may be referred to as a xe2x80x9ccellxe2x80x9d or xe2x80x9ccorexe2x80x9d.
As used herein, xe2x80x9cboundary phasexe2x80x9d is intended to mean a more ductile and/or weaker material that surrounds the cell phase of a monolithic fiber in a relatively thin layer. The boundary phase is disposed between the various individual cell phases, forming a separate layer between the cell phase and surrounding cell phases when a plurality of fibers are formed in a fibrous monolithic composite. The xe2x80x9cboundary phasexe2x80x9d also may be referred to as a xe2x80x9cshell,xe2x80x9d xe2x80x9ccell boundary,xe2x80x9d or xe2x80x9cboundaryxe2x80x9d.
Fibrous monoliths (xe2x80x9cFMsxe2x80x9d) are a unique class of structural ceramics that have mechanical properties similar to continuous fiber reinforced ceramic composites (CFCCs). Such properties include relatively high fracture energies, damage tolerance, and graceful failures. In contrast to CFCCs, FMs can be produced at a significantly lower cost. FMs, which are monolithic ceramics, generally are manufactured by powder processing techniques using inexpensive raw materials. As a result of the high performance characteristics of FMs and the low costs associated with manufacture of FMs, FMs are used in a wider range of applications than heretofore typical for ceramic composites. Thus, FMs are used to form structures having a great variety of shapes and sizes ranging from rather simple essentially two-dimensional structures to very complex three-dimensional structures.
The macroarchitecture of an FM composite generally includes multiple filaments each comprising at least two distinct materialsxe2x80x94a primary phase in the form of elongated polycrystalline cells separated by a thin secondary phase in the form of cell boundaries. The primary or cell phase typically consists of a structural material of a metal, metal alloy, carbide, nitride, boride, oxide, phosphate or silicide and combination thereof. The cells are individually surrounded and separated by cell boundaries of a tailored secondary phase. Powders that may be used in the secondary phase include compounds to create weak interfaces such as fluoromica, and lanthanum phosphate; compounds to create porosity in a layer which function to create a weak interface; graphite powders and graphite-containing powder mixtures; and hexagonal boron nitride powder and boron nitride-containing powder mixtures. If a metallic debond phase is desired, reducible oxides of metals may be used, e.g., nickel and iron oxides, or powders of metals, e.g., nickel, iron, cobalt, tungsten, aluminum, niobium, silver, rhenium, chromium, or their alloys.
Advantageously, powders which may be used in the cell and/or boundary phase composition to provide the green matrix filament include diamond, graphite, ceramic oxides, ceramic carbides, ceramic nitrides, ceramic borides, ceramic silicides, metals, and intermetallics. Preferred powders for use in that composition include aluminum oxides, barium oxides, beryllium oxides, calcium oxides, cobalt oxides, chromium oxides, dysprosium oxides and other rare earth oxides, hafnium oxides, lanthanum oxides, magnesium oxides, manganese oxides, niobium oxides, nickel oxides, tin oxides, aluminum phosphate, yttrium phosphate, lead oxides, lead titanate, lead zirconate, silicon oxides and silicates, thorium oxides, titanium oxides and titanates, uranium oxides, yttrium oxides, yttrium aluminate, zirconium oxides and their alloys; boron carbides, iron carbides, hafnium carbides, molybdenum carbides, silicon carbides, tantalum carbides, titanium carbides, uranium carbides, tungsten carbides, zirconium carbides; aluminum nitrides, cubic boron nitrides, hexagonal boron nitrides, hafnium nitride, silicon nitrides, titanium nitrides, uranium nitrides, yttrium nitrides, zirconium nitrides; aluminum boride, hafnium boride, molybdenum boride, titanium boride, zirconium boride; molybdenum disilicide; lithium and other alkali metals and their alloys; magnesium and other alkali earth metals and their alloys; titanium, iron, nickel, chromium, cobalt, molybdenum, tungsten, hafnium, rhenium, rhodium, niobium, tantalum, iridium, platinum, zirconium, palladium and other transition metals and their alloys; cerium, ytterbium and other rare earth metals and their alloys; aluminum; carbon; lead; tin; and silicon.
Compositions comprising the cell phase differ from those comprising the boundary phase in order to provide the benefits generally associated with FMs. For example, the compositions may include formulations of different compounds (e.g., HfC for the cell phase and WRe for the boundary phase or WC-Co and Wxe2x80x94Nixe2x80x94Fe) or formulations of the same compounds with differing component amounts (e.g., WC-3% Co for the cell phase and WC-6% Co for the boundary phase) so long as the overall properties of the compositions are not the same. For example, the compositions can be selected so that no excessively strong bonding occurs between the two phases in order to limit crack deflection.
The cell boundary phase may be selected to create pressure zones, microcrack zones, ductile-phasezones, or weak debond-type interfaces in order to increase the toughness of the composite. For example, low-shear-strength materials such as graphite and hexagonal boron nitride make excellent week debond-type cell boundaries and are present in Si3N4/BN and SiC/Graphite FM composites. The weak BN and graphite interfaces deflect cracks and determine thereby preventing brittle failure of these composites and increasing their fracture toughness. As a result, FM structures exhibit fracture behavior similar to CFCCs, such as C/C and SiC/SiC composites, including the ability to fail in a non-catastrophic manner.
Fibrous monolith composites are fabricated using commercially available ceramic and metal powders using a process for converting ordinary ceramic powder into a xe2x80x9cgreenxe2x80x9d fiber that include the powder, a thermoplastic polymer binder and other processing aids. Various methods of preparing fibrous monolithic filaments are known in the art, including the methods disclosed in U.S. Pat. No. 5,645,781, which is incorporated by reference herein in its entirety. Generally, the fibrous monolithic filaments that form the composite structures are prepared by first separately blending powders, polymer binders and possibly one or more processing aids as the starting materials for the different phases of the filaments. The materials of the cell and boundary are selected to provide the final structures with predetermined properties. The starting materials are selected from a thermodynamically compatible set of materials available as sinterable powders.
The fiber is compacted into the xe2x80x9cgreenxe2x80x9d state to create the fabric of elongated polycrystalline cells that resemble a fiber after sintering or hot pressing. Once the green composite fiber is fabricated it can be formed using any method known to those skilled in the art into the shape of the desired component having, for example, conventional composite architecture (e.g., uniaxial lay-up, biaxial lay-up, woven fabric, etc.).
In final, finishing processes, the thermoplastic binder is removed in a binder burnout step. The component is sintered to obtain a fully consolidated and densified final structure. The FM composite component is sintered in a pressureless, or essentially pressureless, furnace. The component is heated at temperatures and for a time effective for obtaining a predetermined degree of sintering. The final resultant FM structure has desired properties such as strength, hardness and fracture toughness.
Operating parameters of pressureless sintering are adjusted according to the material characteristics of the particular FM composite being sintered. These parameters are dictated in large part by the melting points of the constituents, their average particle sizes, as well as presence of sintering aids. Gases such as N2 and inert gases such as Ar can be used in the sintering furnace to control the sintering environment. An applied overpressure of these gases (e.g., an overpressure of 6 psi applied in the cold state or an overpressure of 30 psi in a hot state) may be used to improve sintering.
Sintering aids may be blended with one or more of the starting materials to enhance the sinterability of the FM composite. Sintering aids are selected to be physically and chemically compatible with the starting materials while possessing material properties such as lower melting points, higher surface energy and/or higher atomic mobility. In an example of liquid phase sintering, aluminum oxide and yttrium oxide are added to silicon nitride and at the sintering temperature of the system, a low viscosity melt is formed that effectively bonds the silicon nitride grains together. Compositions that may be used as sintering aids include aluminum oxide and yttrium oxide with silicon nitride, silicon carbide with zirconium carbide, zirconium metal with zirconium diboride, and hafnium hydride and carbon with hafnium carbide. The sintering aids are blended in amounts effective for enhancing consolidation and densification of the FM composite during sintering to provide a final FM composite structure with the desired FM properties.
In other embodiments, alternative methods of preparing FM filaments and composite materials may be utilized. Alternative compositions and methods, including those described in the co-pending U.S. patent applications listed in Table 1, which are incorporated by reference herein in their entireties, are contemplated for use with the present invention.